Charmleyjetson:
I am hoping someone can answer this question.My Employer runs Reference Periods 17, 17, 18.
Fixed Week 00:00 Mon - 24:00 Sun
My shift pattern is Thurs-Mon starting at 16:00 and lasts between 10-14 hrs.
If I take a weeks holiday I need to add 48hrs for WTD.
As my shift pattern crosses 2 fixed weeks I can simply add 8 hrs per day which would be 40hrs. But where do I add the additional 8 hrs? Do I add it to the first or second fixed week?
There is no additions 8 hours to add 
For the RT(WT)R 2005 a full weeks leave counts as 48 hours working time but that’s a week that starts and ends at 00:00 Monday, a seven day leave period that crosses two weeks is calculated at 8 hours per days leave.
As your working week is Thursday to Monday you can calculate 40 hours for a working weeks leave.
You may have an agreement with your employer that the a weeks leave crossing two fixed weeks will be calculated at 48 hours but it’s not in the working time regulations.